The present invention relates to a method of making a needle electrode applicable to an electron emitter used for an electron microscope, a CD (critical dimensions) SEM, an electron beam lithography system, an IC tester or the like and an ion source used for a focused ion beam (FIB) source such as a mask repair, an ion implantation device, a device for analyzing a cross-section of a semiconductor device, a specimen preparation apparatus for a transmission electron microscope or the like.
There is a demand for electron emitters of higher brightness in order to increase spatial resolution and efficiency on an electron beam utilizing apparatus such as an electron microscope, and various types of electron emitter such as a thermoionic emitter, e.g., a point filament, or a Schottky emitter have been studied.
For instance, a cold field emitter as a source of emitting electrons having a high brightness wherein a thin wire made of a tungsten single crystal is cut off by electropolishing to form a sharp edge used for an electron emitting surface, is widely used as an electron source for a high resolution electron microscope. Further, a ZrO/W TFE (thermal field emitter) wherein a ZrO layer is coated on the surface of a chip made of a tungsten single crystal so that the work function of (100) surface is reduced from about 4.5 eV to about 2.8 eV is noted in recent years.
Even in any electron emitter, as understood from the Fowler-Nordheim formula or the Schottky formula, the emission current density is determined based on the work function of the electron emitting area of the emitter and a distribution of the electric field strength. The distribution of the electric field strength depends strongly on a voltage applied across the emitter and the extraction electrode and the geometry of the emitting area located at the end of the emitter in particular, and it is an important factor controlling the characteristic of the electron emitter.
Regarding the geometry of the top end of an electron emitter, for instance, the before-mentioned ZrO/W TFE, there has been known that when the radius of curvature of the top end is increased to 1 .mu.m or more, the energy spread of emitting electrons is decreased, and stability is improved (J. Vac. Sci. Technol., 16(6) 1979, 1704-1708, J. Applid Physics., 46, 5(1975) 2029-2050).
Further, Japanese Unexamined Patent Publication JP-A-7-105834 discloses a method that an electric discharge makes the curvature of the radius in TFE larger so that the energy spread becomes small and a stable emission is obtainable. It also discloses that in trying to heat the conventional TFE to about 2,800 K, it has been found that the half cone angle is large as 40.degree. or more which is unsuitable for practical use. In addition, the publication suggests that a TFE having a large radius of curvature and having a shape of the top end in which the half cone angle is small can not be processed by the conventional electropolishing method.
In fact, in the conventional electropolishing method, the half cone angle becomes large as the radius of curvature is larger, and it was difficult to control the half cone angle to be 5.degree. or less even when the radius of curvature was 0.6 .mu.m or less, or to control the half cone angle to be 10.degree. or less even when the radius of curvature was 0.6 .mu.m-2.0 .mu.m.
In Japanese Unexamined Patent Publication JP-A-8-36981, the inventors of this application propose that a TFE having a large radius of curvature of 1.2-10 .mu.m and a full cone angle of 25.degree. or less can provide in a stable manner electron beams having an energy spread of 0.5 eV or less and a angular current density of 0.02 mA/sr or more at a rate of change of 5% or less. Further, they disclose that the above-mentioned TFE is obtainable by combining a dry-etching method with the conventional electropolishing method.
However, the method disclosed in the publication involves a problem that an electron emitter having a desired shape of the top end, in particular a shape of the top end wherein the radius of curvature is 0.6 .mu.m or more and the half cone angle is 10.degree. or less can not be obtained at a reduced cost since the method utilizes processes of low productivity such as electric discharging and dry etching, and further, an expensive device for inclusive use is needed.
A focused ion beam (FIB) source is used for various types of semiconductor inspection apparatus and semiconductor processing apparatus, and it attracts users attention in recent years. In particular, a liquid metal ion source wherein gallium is used as ion species is widely known as an ion source for FIB.
The liquid metal ion source is so adapted that a needle electrode made of metal having a high melting point is gotten wet with liquid metal, and a high electric field strength is applied to a sharp edge of the needle electrode to ionize the liquid metal. A cone-like protection of liquid metal which is called Tailor cone is formed at the sharp edge of the needle electrode by the effect of the high electric field strength, and ions are emitted from the sharp edge. The cone angle of the Tailor cone is supposed to be about 97.degree. in full angle, and it is known to be important to form the cone angle at the sharp edge of the needle electrode in conformity with the cone angle of Tailor cone.
Accordingly, the sharp edge is generally formed by an electropolishing method in the same manner as in the needle electrode for the electron emitter. Besides use of the electropolishing method, a technique of mechanically polishing can be used. However, such mechanically polishing method requires a special jig because it is used for a fine part, with the result that manufacturing cost becomes high. Further, the electropolishing method can not easily provide a needle electrode having a cone angle which is close to the cone angle of Tailor cone in stable manner.
The electropolishing method for the needle electrode is disclosed in, for example, "Kotai Butsuri", vol. 2, No. 2 (1966) 33-38. However, the conventional method has a problem that the cone angle and the radius of curvature of the top end portion can not independently be controlled because they are in a strong correlation, in particular, when the radius of the top end is large, it is difficult to reduce the half cone angle.